Non-rising stem valves for use in distribution systems are known in the prior art. These non-rising stem valves are provided with a stem packing which, by compression and internal pressure, effects a seal and contains the product under pressure therein. To replace this stem packing, the valve and the subsequent distribution system must be taken out of service to avoid any major loss of product. One solution to the problem of downtime is set forth in U.S. Pat. No. 3,912,221 to Fenster et al issued Oct. 14, 1975, which discloses a stem seal and retainer for a gate valve. This gate valve is of the non-rising type with a seal being provided between the stem and bonnet of the valve. A cartridge type seal unit is held in place by the operating nut and the stem and gates are held in the raised position by a snap ring. The cartridge type seal unit may be removed by removing the operating nut and lifting out the seal cartridge.
The non-rising valve described above includes a secondary seal to prevent loss of product while the primary seal is being replaced. This allows the primary seal to be replaced while the valve is in the operating condition. This redundant secondary O-ring seal is continuously in effect by way of compression between the non-rising stem and the bonnet, and internal pressure exerted by the product line. These forces which continuously act on the secondary O-ring seal subject this seal to constant wear during the opening and closing of the valve, and when such secondary seal requires replacement due to the stresses which are continuously applied thereto, the valve and distribution system must be decommissioned to avoid loss of product during this replacement.